1. Field of the Invention
The present invention relates to a pneumatic radial tire, and more specifically to an improvement in a pneumatic tire formed with a block pattern on the tread thereof.
2. Description of the Prior Art
In general, pneumatic tires called all-weather or studless tires are formed with a great number of blocks partitioned by a plurality of main circumferential grooves extending in the tire circumferential direction and arranged with regular intervals in the axial direction of the tread, and a plurality of transverse grooves extending to cross these main grooves at angles with respect to the main circumferential grooves and arranged at regular intervals in the circumferential direction of the tread.
In order to improve the running performance on dry or snow and ice roads, the studless tire is formed with a number of sipes on each block. In particular, by dividing each block into smaller blocks by these sipes, the block rigidity is lowered to improve the adherence characteristics to ground and further to increase the block edge effect or ground holding effect based upon the circumferential ridge lines extending in the tire rotating direction of the block.
In the prior-art pneumatic tire of this kind, however, since the depth of these sipes formed in each block is substantially the same in each block, there exists a problem in that both the side ends of each block (when seen along the vehicle travel direction) easily wear away when the tire is rotated under heavy load. This results in biased abrasion in each block in the circumferential direction of the tire. As a result, the adherence characteristics to ground are deteriorated, the edge effects on both block side ends are reduced; and the traction and brake performance required for studless tires are degraded.